3 EU Floods Directive FLOOD HAZARD MAPS AND FLOOD RISK MAPS Article 6 7. Member States may decide that, for areas where flooding is from groundwater sources, the preparation of flood hazard maps shall be limited to the scenario referred to in paragraph 3(a) 3. Flood hazard maps shall cover the geographical areas which could be flooded according to the following scenarios: (a) floods with a low probability, or extreme events scenarios; (b) floods with a medium probability (likely return period 100 years); (c) floods with a high probability, where appropriate.

4 Groundwater flooding Groundwater flooding is the emergence of groundwater at the ground surface away from perennial river channels, or the rising of groundwater into manmade ground under conditions, where the normal ranges of groundwater level and groundwater flow are exceeded.

6 Risk from groundwater flooding estimated 1.6M properties at risk in England and Wales (Jacobs, 2004) most vulnerable properties 383,000 located on exposed chalk aquifers Patcham area of Brighton 2000/1-800K (not including impact on rail network) under reporting: not clear its groundwater only seek assistance if water levels are significantly deep reporting may affect the value of the property

8 Groundwater flooding in unconfined aquifers primarily the Chalk of south and east England response to prolonged extreme rainfall coupled with above average antecedent groundwater levels flooding may occur a few days or several weeks after major recharge event and often lasts for weeks generally in upper reaches of Chalk catchments in localised low topography due to upstream extension of stream sources and emergence of rarely activated springs

9

10 Chalk: dual porosity Chalk diagram fissures matrix

11 Groundwater flooding mechanism: Chalk aquifer During periods of rapid recharge, low storage fissure system takes up majority of recharge causing rapid rise in groundwater levels Rise particularly large away from valley bottoms where fissure network is less developed Tens of metres of rise during enhanced recharge season Low/moderate permeability means elevated heads do not readily dissipate Groundwater released from matrix slows recession

16 Groundwater flooding in alluvial aquifers relatively common compared with unconfined aquifer flooding occurs when water moves laterally out through the permeable sides of a river channel into lower lying alluvial deposits often precedes fluvial flooding particularly where engineering measures allow high in-bank river levels more likely where relatively narrow floodplains Complicated by urban drainage and man-made structures relatively short-lived flooding as high permeability sediments allow groundwater to drain once river levels fall

22 Groundwater flooding: management responsibility Environment Agency remit to provide warnings for fluvial, tidal and coastal flooding only 2004, Defra s Making Space for Water strategy promotes an holistic approach to managing flood risk From Spring 2006 EA assumed strategic overview for monitoring groundwater flooding but extent of role currently being clarified Series of studies ongoing to assess whether flood warning and risk monitoring can incorporate other sources of flooding and issues surrounding monitoring and records collation in relation to groundwater flooding

26 Chalk: approach recommended to EA Jacobs, JBA (2006, 2007) Understanding not sufficient to calculate return period - recorded incidents currently most accurate form of hazard mapping All existing groundwater flood data to be collated and future collection of data done in a consistent manner Warnings targeted at communities thought to be at risk, based on trigger levels in key boreholes

27 Alluvial aquifers: approach recommended Jacobs, JBA (2006, 2007) For many localities within Extreme Flood Outline (EFO) assumption is they are served by fluvial flood warning service Relatively straightforward to map and then incorporate those areas outside the EFO that are below the 1% flood level and are in hydraulic continuity Recommended using current fluvial flood warning service build in groundwater level monitoring Current groundwater monitoring network insufficient but relatively cheap to install

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